Electrochemical Reduction of Hexahydro-1,3,5-trinitro-1,3,5-triazine in Aqueous Solutions PASCALE M. L. BONIN, † DORIN BEJAN, † LEAH SCHUTT, † JALAL HAWARI, ‡ AND NIGEL J. BUNCE* ,† Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1, and Biotechnology Research Institute, National Research Council of Canada, 6100 Royalmount Avenue, Montreal, Quebec, Canada H4P 2R2 Electrochemical reduction of RDX, hexahydro-1,3,5- trinitro-1,3,5-triazine, a commercial and military explosive, was examined as a possible remediation technology for treating RDX-contaminated groundwater.A cascade of divided flow-through cells was used, with reticulated vitreous carbon cathodes and IrO 2 /Ti dimensionally stable anodes, initially using acetonitrile/water solutions to increase the solubility of RDX. The major degradation pathway involved reduction of RDX to the corresponding mononitroso compound, followed by ring cleavage to yield formaldehyde and methylenedinitramine. The reaction intermediates underwent further reduction and/or hydrolysis, the net result being the complete transformation of RDX to small molecules. The rate of degradation increased with current density, but the current efficiency was highest at low current densities. The technique was extended successfully both to 100% aqueous solutions of RDX and to an undivided electrochemical cell. Introduction Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX, structure 1 ) is a widely used commercial and military explosive (1). During its manufacture, process wastewater (“pink water”) may be discharged to the environment at concentrations up to 12 m g/ L RDX (2). Plumes of RDX have developed from pink water lagoons (3), and from military testing, training, waste disposal, and demilitarization operations (4, 5). RDXand its degradation products are reported to be toxic, mutagenic, and carcinogenic (6, 7); as a result there is a need for remediation of contaminated sites, as well as treatment of process wastewater prior to discharge. The decomposition of RDXcan be initiated chemically (8, 9), photochemically (10, 11), biologically (12-17), or electrochemically (18, 19). Although mechanisms of decom- position are not known with certainty, postulated sites of attack generally involve one of the nitro groups of RDX. McCormick et al. (12) proposed that the degradation ofRDX in municipal anaerobic sludge occurred via successive reduction of the nitro groups to give hexahydro-1-nitroso- 3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso- 5-nitro-1,3,5-triazine (DNX),and hexahydro-1,3,5-trinitroso- 1,3,5-triazine (TNX). Subsequent fragmentation of the ring was proposed as the route to the end products hydrazine, 1,1-dimethylhydrazine, 1,2-dimethylhydrazine, formalde- hyde,and methanol.More recently,Hawariet al.(13)reported the identification of an intermediate ring cleavage product, methylenedinitramine [MDNA, CH2(NHNO2)2], which was detected in almost stoichiometric amounts under anaerobic conditions. This suggested a different degradation pathway, in which RDXunderwent ring cleavage via MNXor via direct enzymatic attack (14), in place of successive reductions of the nitro groups. Nitrous oxide and formaldehyde, which further biotransformed to CO2, were identified as end products. Electrolysisisa developingtechnologyforthe remediation ofindustrialwastes,and hassuccessfullybeen applied to the remediation of explosives (18-20). Rodgers et al. (20) demonstrated electrochemical reduction of 2,4,6-trinitro- toluene (TNT)at a reticulated vitreouscarbon (RVC)cathode in high current efficiency and high chemical yield. A subsequent oxidative treatment was needed to remove the reduction products, anilines, by precipitation (20). Electro- chemicaloxidation (18)and reduction (19)ofRDXhave been reported, but in both cases, degradation of RDXwas based only on the loss of a chromatographic response and the reaction products were not identified. The aim of the present investigation was to develop an electrochemical method for the remediation of RDX- contaminated water under reductive conditions. Specific goals included finding conditions to optimize the disap- pearance ofRDXand its potentially toxic reaction products, product identification, and the determination of chemical and current yields. Experimental Section Chemicals. RDXwas synthesized in our laboratoryfollowing a procedure derived from Hale (21).Itsidentitywasconfirmed by chromatographic comparison to an analytical standard (g98% pure) from Chem Service, Inc. and by melting point analyses (mp 205.5 °C and mmp 208 °C). The synthesized RDX had a purity of 98% by HPLC. MNX and TNX were provided by Defense Research and Development Canada (DRDC),Valcartier,Canada.Methylenedinitramine (MDNA) was obtained from the Rare Chemical Library of Aldrich, Canada. All other chemicals used were of reagent grade. HPLC Analysis. The equipment comprised a Waters model600pump (flowrate 1.0mL/min),a Rheodyne injector containing a 20 μL sample loop, and a Waters model 486 tunable absorbance detector operated at 240 nm. The detector output was processed with Waters Millennium version 3.2 software. Separation of RDX, MNX, TNX, and MDNA was achieved on a Genesis C18 (25 cm × 4.6 mm) stainless steel column, using a 65:35 volumetric solution of acetonitrile (ACN)/H2Oasthemobilephase.Allsolventsused were Fisher Scientific HPLCgrade.Analyses for formaldehyde involved mixing the aqueous solution with an equalvolume ofNash reagent (22),resultingin the development ofa yellow *Corresponding author phone: (519) 824-4120, ext 53962; fax: (519) 766-1499; e-mail: bunce@chembio.uoguelph.ca. † University of Guelph. ‡ National Research Council of Canada. Environ. Sci. Technol. 2004, 38, 1595-1599 10.1021/es0305611 CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1595 Published on Web 01/28/2004